The New York Times recently published an article bemoaning the expected end of marginalia. In addition the rise of e-books, they also point to the trend towards making books too sacred to mar with personal notes.
I admit, that I am one of those who finds himself unable to write marginalia on a regular basis. Sometimes, I make a particular effort to do so, but it does not come naturally.
Nevertheless, I will join them in mourning the death of marginalia, if it comes. Recently, I saw an exhibition at the Folger Shakespeare Library on the marginalia seen in books in their collection and it was a wonderful and eye opening experience.